As part of its effort to standardize the interface of the electronic equipment, the electronic industry has established pin assignments for some of the connectors that will be used in the interface. For various computer applications, it is desirable to interconnect memory cards. A sixty-eight pin connector, for example, is often used. This connector has sixty signal pins, four ground return pins and four DC voltage pins, all of which are preassigned by the industry. The signal to ground ratio of an electrical connector is equal to the number of signal carrying pins divided by the number of ground return pins in the connector. For the purposes of the present disclosure the DC voltage pins can be considered similar in effect to the AC voltage ground pins, thus the signal to ground ratio of the sixty-eight pin connector is 7.5 to 1.0. Typically in computer applications multiple lines are simultaneously switched and all return current generated by this switching must be returned through one of the ground pins. The return current of eight or so signal pins, therefore, must be accommodated by a single ground pin. When the signal rise time is relatively slow, in the eight to ten nanosecond range, this presents no problem. When the rise time is increases, however, as in certain computer applications the induced voltage is increased resulting in "ground bounce" or common mode noise in the ground return pins. When the ground bounce reaches a high enough level, relative to the level of the signals, the systems may become unable to reliably read and respond to the signals thereby causing what is known in the industry as "false triggering". Since the pin assignments have been fixed by the industry, the signal to ground ratio cannot be altered. To reduce the adverse effects of the faster rise times a conductive shroud may be used to electrically interconnect the ground of the memory card to the ground of the equipment with which the card is being used.
One such shroud and a related connector are disclosed in U.S. Pat. No. 5,288,247, issued Feb. 22, 1994, to Kaufman. This patent is hereby incorporated by reference. The shroud of the '247 patent is arranged to enclose the top of the connector and two sides thereof. A shroud that will accept multiple connectors in a vertically stacked relationship that benefit from the improved performance of the single connector shroud is disclosed in U.S. Pat. No. 5,399,105 and also owned by the present assignee. This application is hereby incorporated by reference. The shrouds in the above two references include contacts that extend from the edge of the shroud and electrically engage a conductive outer surface of a memory card that is mated to the connector. Each of the shrouds is electrically connected to ground circuitry on a circuit board. The performance of the equipment is greatly improved by the use of such shrouds. The shrouds and connectors in the above references further retain the capacity to connect with existing memory cards that do not have conductive outer surfaces, nor grounding circuitry within the cards. A problem arises, however, with some of the older cards that use conductive covers but do not have the required ground interconnections within the memory card. Use of an improperly grounded card with the shrouds disclosed in the '247 patent or the '105 patent for example could cause electromagnetic interference. What is needed, therefore, is a connector and shroud assembly that will establish electrical connection to a properly grounded memory card while avoiding electrical connection with a card that has not been properly grounded, regardless of whether the card has an insulative or a conductive cover.